An organic electroluminescence (EL) device may be a fluorescent organic EL device or a phosphorescent organic EL device, and an optimal device design for the emission mechanism of each type of organic EL device has been studied. It is known that a highly efficient phosphorescent organic EL device cannot be obtained by merely applying fluorescent device technology due to the emission characteristics. The reasons therefor are generally considered to be as follows.
Specifically, since phosphorescence utilizes triplet excitons, a compound used for forming the emitting layer must have a large energy gap. This is because the energy gap (hereinafter may be referred to as “singlet energy”) of a compound is normally larger than the triplet energy (i.e., the difference in energy between the lowest excited triplet state and the ground state) of the compound.
Therefore, it is necessary to form the emitting layer using a host material having a triplet energy higher than that of a phosphorescent dopant material in order to efficiently confine the triplet energy of the phosphorescent dopant material in the emitting layer. It is also necessary to provide an electron-transporting layer and a hole-transporting layer adjacent to the emitting layer, and form the electron-transporting layer and the hole-transporting layer using a compound having a triplet energy higher than that of the phosphorescent dopant material.
Specifically, since a compound having a large energy gap as compared with a compound used for the fluorescent organic EL device is necessarily used for the phosphorescent organic EL device when designing the phosphorescent organic EL device based on a known organic EL device design concept, the driving voltage for the entire organic EL device increases.
Hydrocarbon compounds that exhibit high oxidation resistance and high reduction resistance and are useful for the fluorescent device have a small energy gap due to the large spatial extent of the π-electron cloud. Therefore, an organic compound that contains a heteroatom (e.g., oxygen or nitrogen) is selected for the phosphorescent organic EL device instead of such hydrocarbon compounds. As a result, the phosphorescent organic EL device has a short lifetime as compared with the fluorescent organic EL device.
The fact that the relaxation rate of triplet excitons of a phosphorescent dopant material is much higher than that of singlet excitons also significantly affects the device performance. Specifically, it is expected that efficient emission from singlet excitons can be obtained since diffusion of excitons into the layers (e.g., hole-transporting layer and electron-transporting layer) situated around the emitting layer rarely occurs due to the high relaxation rate that leads to emission. In contrast, since emission from triplet excitons is spin-forbidden (i.e., the relaxation rate is low), diffusion of excitons into the layers situated around the emitting layer easily occurs, and thermal energy inactivation occurs from a compound other than a specific phosphorescent compound. Specifically, it is important to control the electron-hole recombination region as compared with the fluorescent organic EL device.
It is thus necessary to select materials and a device design differing from those of the fluorescent organic EL device in order to obtain a highly efficient phosphorescent organic EL device.
In particular, when designing a phosphorescent organic EL device that emits blue light, it is necessary to use a compound having high triplet energy for forming the emitting layer and the layers situated around the emitting layer as compared with a phosphorescent organic EL device that emits green to red light. More specifically, the triplet energy of a host material used to form the emitting layer generally must be 3.0 eV or more in order to obtain blue phosphorescence without causing efficiency loss. In order to obtain a compound that has such a high triplet energy and meets the requirements for an organic EL material, it is necessary to employ a new molecular design concept that takes account of the electronic state of π-electrons instead of merely combining molecular parts (e.g., heterocyclic compound) having high triplet energy.
In view of the above situation, a compound having a structure obtained by bonding a plurality of heterocyclic rings has been studied as a material for a phosphorescent organic EL device that emits blue light. For example, JP-A-2009-021336 discloses a compound having a dibenzofuran ring and an azine ring as a material for forming an electron-transporting layer.
WO2008/072596 discloses a compound obtained by bonding two dibenzofurans and the like via a divalent linking group as a host material for forming a phosphorescent emitting layer.
JP-A-2011-084531 discloses a compound having an azadibenzofuran structure as a host material for forming a phosphorescent emitting layer and a material for forming an electron-transporting layer.